--branch-probabilities should be optional

Description

Whether or not to use the --branch-probabilities option with gcov should be optional. We've turned it off and are only using line coverage because C++ compilers typically create many extra branches that may not be worth getting coverage for.

Basically, we updated the gcovr script to not pass the "--branch-probabilities" option to gcov, which has the effect of only reporting line coverage and not branch coverage. In other words, we don't want branch coverage. Initially we did, but then we realized that it was confusing with C++ since the compiler adds extra branches. Does that make more sense?